"MM" wrote in message
...


And there's no point having a lockable cap, because they just punch a
hole in the tank.

No you need to do what Chubb used to do when I worked there.. run a
continuous length of thin, weak wire in a snake pattern all over the surface
yo want to protect and run the alarms anti-tamper circuit through it. Any
attempt to get through breaks the wire and sounds the alarm. You will
probably have to glue the cable to the tank or use the lead foil strips that
you see on shops. The foil is easy to attach using varnish.